中国腐蚀与防护学报, 2017, 37(3): 293-299
doi: 10.11902/1005.4537.2016.030
7075厚板铝合金搅拌摩擦焊接头晶间腐蚀行为研究

Intergranular Corrosion Behavior of Friction-stir Welding Joint for 20 mm Thick Plate of 7075 Al-alloy
刘德强1,2, 柯黎明1,3,, 徐卫平1, 邢丽1, 毛育青3

摘要:

对20 mm厚7075铝合金搅拌摩擦焊 (FSW) 接头沿板厚方向进行分层晶间腐蚀行为研究。借助光学显微镜及扫描电子显微镜分析了接头组织、第二相成分及分布、腐蚀深度及接头各区腐蚀形貌。结果表明:焊缝中心区腐蚀程度最轻,热机影响区 (TMAZ) 次之,热影响区 (HAZ) 腐蚀程度最严重;沿板厚向下,焊核区 (NZ) 腐蚀程度逐渐变大,TMAZ腐蚀程度先变大后减小,HAZ腐蚀程度逐渐减小;接头沿板厚方向晶粒大小和第二相粒子尺寸及分布存在差异,是造成沿板厚方向各区不同晶间腐蚀程度的主要原因。

关键词: 搅拌摩擦焊 ; 厚板 ; 7075铝合金 ; 晶间腐蚀 ; 第二相

Abstract:

The intergranular corrosion behavior of friction-stir welding (FSW) joint for 20 mm thick plate of 7075 Al-alloy was investigated in solution of 57 g NaCl+1000 mL H2O+10 mL H2O2 corresponding to the national standard GBT7998-2005. The microstructure of welded joint, the composition and distribution of the second phase, corrosion depth and corrosion morphology of different zones of the weld joint were characterized by optical microscope and scanning electron microscope. The results show that the corrosion severity of the center zones of the weld joint is the lightest. The corrosion severity of the thermal mechanical affected zone (TMAZ) is between that of the center zones and heat affected zone (HAZ). Downward along the thickness direction of the weld joint, the corrosion severity of the nugget zone (NZ) gradually increased, while that of the TMAZ increased first and then decreased, and that of the HAZ gradually decreased. The difference in the size and the distribution of the second phase and in the grains size of the weld joint along the thickness direction may be the main factor which caused the different corrosion severity for different zones along the thickness direction of the weld joint.

Key words: FSW ; heavy plate ; 7075 Al-alloy ; intergranular corrosion ; the second phase

7075铝合金具有较高的比强度、弹性模量、断裂韧性[1],因此广泛用于制造飞机的梁、起落架零件等。随着航天航空技术的兴起,对于厚板铝合金在航空工业上的应用变得越来越广泛。1991年发明的搅拌摩擦焊 (FSW)[2]能很好的解决铝合金在熔化焊时产生的焊缝金属凝固裂纹、热影响区软化及应力集中[3]等常见问题,因此在航空领域得到了广泛的应用。

目前,对于厚板铝合金FSW接头性能的研究主要集中在接头的力学性能[4-9],但关于厚板铝合金FSW接头沿厚度方向的腐蚀行为的报道不多。厚板铝合金在FSW时,在板厚方向存在大的温度梯度,导致在焊接过程中焊缝厚度方向金属塑化程度和流动行为存在差异,造成组织结构不同,进而使得接头沿板厚方向存在腐蚀性能差异。Xu等[10]对14 mm厚的2219-O铝合金FSW接头焊核区上中下3个位置的点蚀行为进行了研究,焊核上部区域相比于下部区域表现出了更好的耐蚀性能。胡百晖等[11]对8 mm厚的LY12铝合金FSW焊缝进行分层腐蚀,研究结果表明,沿厚度方向向下,焊核区腐蚀程度逐渐变大,而热机影响区 (TMAZ) 和热影响区 (HAZ) 的腐蚀程度逐渐减小。

对于铝合金材料来说,晶间腐蚀是常见的腐蚀形式之一,而且腐蚀前期不容易被发现,给使用这些材料的飞机和船舶带来了严重的安全隐患。目前对于厚板铝合金FSW焊缝的晶间腐蚀行为的研究较少。因此,本文以20 mm厚的7075铝合金板材为研究对象,沿焊缝厚度方向对FSW接头进行分层晶间腐蚀,研究各层各区域的晶间腐蚀行为,分析组织结构对厚板7075铝合金FSW接头晶间腐蚀行为的影响。

1 实验方法

实验材料为20 mm厚的AA7075-T6铝合金板材,其化学成分 (质量分数,%) 为Si 0.1,Zn 5.43,Mn 0.06,Ti 0.03,Fe 0.36,Cu 1.6,Cr 0.2,Mg 2.7,Al余量。实验选定在自制的搅拌摩擦焊设备上进行。选用的搅拌头轴肩36 mm,锥形的搅拌针端部直径8 mm、根部直径14 mm、长19.7 mm。FSW工艺参数为:旋转速率235 r/min,焊接速率30 mm/min,倾斜角度2°,下压量0.5 mm。

沿焊件水平面截取金相试样,打磨、抛光后,用Keller试剂腐蚀,采用Zeiss Axioscope A1型光学显微镜对接头各区进行微观组织观察。

晶间腐蚀试样截取示意图见图1。将切好的50 mm×10 mm×20 mm的接头沿厚度方向每隔4 mm切成1个50 mm×10 mm×4 mm的试样,共计5个试样,并从上到下将5个试样分别标号为1#,2#,3#,4#和5#。将试样的上表面打磨、抛光,然后按GBT7998-2005铝合金晶间腐蚀测定方法进行晶间腐蚀实验。腐蚀介质为57 g NaCl+1000 mL H2O+10 mL H2O2溶液。选取各层上表面为腐蚀面,非腐蚀面进行密封处理;腐蚀温度为 (35±2) ℃,腐蚀时间为6 h。腐蚀后,取20 g CrO和50 mL H3PO4加水稀释到1000 mL后,加热到80 ℃,然后将腐蚀试样浸泡5~10 min,去除腐蚀产物。采用光学显微镜测量晶间腐蚀深度,用Canon EOS 50D单反相机对试样腐蚀宏观形貌进行观察,用VEGA II LMH型扫描电子显微镜 (SEM) 及自带能谱仪 (EDS) 对试件各区微观腐蚀形貌进行观察及各区域内第二相粒子EDS分析。

图1 晶间腐蚀试样截取示意图

Fig.1 Schematic diagram of cutting of samples for intergranular corrosion tests

2 结果与讨论
2.1 腐蚀宏观形貌

图2为接头横截面宏观形貌图。根据组织特征形貌可以将接头分为4个区域:焊核区 (NZ)、热机影响区 (TMAZ)、HAZ和轴肩影响区 (SAZ)。图中AS为前进边,RS为返回边。本文将NZ和SAZ视为焊缝中心区。

图3是沿板厚方向各层试样上表面晶间腐蚀宏观形貌。从图3并且参照图2各区位置可见,HAZ因存在大量腐蚀坑,对光的漫反射作用增强而颜色明显偏暗,说明腐蚀程度严重,而焊缝中心区和TMAZ的颜色比较白亮,说明腐蚀程度较轻。图3a为焊缝上表面的SAZ,可见大量的腐蚀坑,表明其腐蚀程度较下部NZ严重。

图2 7075铝合金焊接接头横截面宏观形貌

Fig.2 Macroscopic morphology of cross section of FSW joint of 7075 Al alloy plate

图3 沿板厚方向各层试样上表面晶间腐蚀宏观形貌

Fig.3 Intergranular corrosion morphologies of the top surfaces of 1# (a), 2# (b), 3# (c), 4# (d) and 5# (e) samples locating at different depths of FSW joint

2.2 腐蚀微观形貌

图4为3#试样各区晶间腐蚀显微形貌。可以看出,不同的区域表现出不同的腐蚀行为,NZ腐蚀程度最轻,HAZ腐蚀最严重。由图4b和c可见,NZ分布大量点蚀坑,且出现了明显的晶间腐蚀。由图4e可见,HAZ部分区域表层晶粒发生了大量脱落,出现了严重的剥蚀现象,同时可见大量腐蚀坑分布其上,表明HAZ不仅出现了晶间腐蚀,还伴随着点蚀和剥蚀现象。由图4f可见,晶粒晶界和晶粒内部都遭受了严重的腐蚀,逐渐向基体内部腐蚀。由图4h可见,TMAZ区存在大量不规则的腐蚀坑,腐蚀坑内仍有未脱落的第二相粒子 (如图中箭头所示)。由于晶界被腐蚀,可以从图4i观察到特征明显的TMAZ晶粒组织。因此,TMAZ内晶间腐蚀和点蚀同时存在。

图4 3#试样各区晶间腐蚀显微形貌

Fig.4 Intergranular corrosion microstructures of different zones of 3# sample: (a) NZ, (b, c) magnified images of areas A and B in Fig.4a and Fig.4c, respectively; (d) HAZ, (e, f) magnified images of areas C and D in Fig.4d and Fig.4e, respectively; (g) TMAZ, (h, i) magnified images of the areas E and F in Fig.4g and Fig.4h, respectively

2.3 晶间腐蚀深度

表1为沿焊缝厚度方向各层试样各区晶间腐蚀深度的测量值。可知,焊缝中心区域的SAZ和NZ腐蚀深度最小,HAZ腐蚀深度最大,这与宏观和微观腐蚀形貌结果一致。焊缝中心最大腐蚀深度出现在1#试样的SAZ,腐蚀深度为23.8 μm。图5为3#试样各区晶间腐蚀深度测量图。由图5b可见,TMAZ腐蚀后形貌呈波浪状,这与TMAZ扭曲变形的组织结构相一致,也从侧面印证了腐蚀沿晶间进行。从表1可知,TMAZ的最大腐蚀深度出现在焊缝中部的3#试样上,最大腐蚀深度为68.5 μm。结合图5c可知,HAZ表现出最严重的腐蚀倾向,最大腐蚀深度出现在焊缝最上部的1#试样的HAZ上,深度为123.8 μm。

表1 沿板厚方向试样各层各区晶间腐蚀深度
Table 1 Intergranular corrosion depths of different zones of three samples locating at different depths of FSW joint
Sample Area Max. depth / μm
1# NZ 23.8
HAZ 123.8
TMAZ 44.4
3# NZ 13.3
HAZ 80.0
TMAZ 68.5
5# NZ 18.8
HAZ 71.1
TMAZ 57.8

表1 沿板厚方向试样各层各区晶间腐蚀深度

Table 1 Intergranular corrosion depths of different zones of three samples locating at different depths of FSW joint

图5 3#试样各区晶间腐蚀深度测量图

Fig.5 Corrosion depth maps of NZ (a), TMAZ (b) and HAZ (c) of 3# sample

2.4 接头微观组织及第二相分布

图6为沿焊缝厚度方向水平面不同区域的微观组织形貌。由图6a~c可见,焊缝中心区为细小的等轴晶,沿焊缝向下晶粒尺寸略微变大。这是因为焊缝中心上部金属材料受到轴肩的挤压摩擦和搅拌针的双重摩擦搅拌作用,温度最高,晶粒发生了充分的动态再结晶,所以晶粒最为细小;沿板厚向下,温度和搅拌作用逐渐减弱,晶粒动态再结晶程度没有上部那么充分,所以晶粒尺寸逐渐变大。由图6d~f可见,TMAZ组织发生严重变形,在垂直于轧制方向呈弯曲的长条状,晶粒尺寸沿板厚方向,先变大后变小。这是因为TMAZ上部组织由于受到的温度较高,塑性挤压作用较强,所以部分晶粒发生再结晶变成细小的等轴晶;TMAZ中部组织塑性流动较为强烈,晶粒为弯曲的长条状,晶粒尺寸较大;TMAZ下部范围较窄,紧贴NZ,受到搅拌针带动的塑性金属挤压作用,晶粒为长条状和不规则块状,晶粒尺寸略小于中部的。由图6g~i可见,HAZ沿板厚方向晶粒尺寸逐渐变小,这是因为沿板厚方向温度逐渐降低[12],HAZ组织粗化程度减弱。图7为1#试样各区第二相分布,表2为第二相粒子成分分析。可见,3个区域的第二相粒子形貌都呈不规则块状。NZ第二相粒子尺寸最小,分布最均匀,尺寸约为1~10 μm;TMAZ中第二相粒子分布均匀性较差,尺寸约为1~15 μm;HAZ中第二相粒子分布最不均匀,成串排列,尺寸约为5~20 μm。由表2的粒子成分分析可以确定,这些不规则块状第二相粒子可能是由7075铝合金在铸锭凝固过程中形成的FeCrAl7和FeAl3等粗大难溶相和FSW过程的高温析出相共同组成的。

分析认为,工业使用的7075铝合金在铸锭凝固过程中会形成FeCrAl7和FeAl3等粗大难溶相,在随后的加工过程中被破碎,往往排列成串[13,14]。当厚板7075铝合金在FSW时,由于焊缝中心区搅拌针的机械作用强,这些难溶第二相发生破碎,尺寸减小,并随着金属塑性流动分布较为均匀,所以NZ第二相粒子尺寸最小;HAZ只受到热循环的作用,第二相粒子发生粗化,所以尺寸最大;TMAZ介于NZ和HAZ之间,同时又受到流动金属的挤压作用,第二相部分发生破碎,但效果远远弱于NZ中的,因此该区第二相尺寸和分布均匀性介于NZ和HAZ中的之间。

图6 1#,3#和5#试样沿焊缝厚度方向水平面不同区域微观组织形貌

Fig.6 Microstructures of different zones of 1# (a, d, g), 3# (b, e, h) and 5# (c, f, i) samples along the weld thickness direction: (a) SAZ, (b) top of NZ, (c) bottom of NZ, (d) top of TMAZ, (e) middle of TMAZ, (f) bottom of TMAZ, (g) top of HAZ, (h) middle of HAZ, (i) bottom of HAZ

图7 1#试样各区第二相分布

Fig.7 Distributions of second-phase particles in different zones of 1# sample: (a) NZ, (b) TMAZ, (c) HAZ

2.5 分析与讨论

铝合金发生晶间腐蚀的主要原因是在晶界存在连续析出相和无沉淀析出带[15],铝合金中第二相往往优先在晶界处析出,造成晶界附近化学溶质偏聚,产生电位差,发生电化学反应。由于晶粒越大,在晶界析出的第二相粒子越多,晶界处发生沿晶腐蚀的倾向就越大[16],并且大尺寸晶粒,有利于腐蚀介质通过连续晶界向金属深层发展。对于本研究中的7075铝合金FSW接头,焊缝中心区组织为细小的等轴晶,所以晶间腐蚀倾向最小,而HAZ受热循环影响组织发生粗化,晶间腐蚀倾向最大。

在焊缝中心沿板厚向下,晶粒尺寸逐渐变大,增大了晶间腐蚀倾向。但从表2可知,1#试样SAZ的晶间腐蚀深度大于焊缝下部的3#和5#试样NZ的晶间腐蚀深度。这是因为焊缝上表面SAZ的金属由于受到搅拌头反复挤压,形成周期性弧纹结构,大量的第二相粒子也呈周期性聚集在弧纹的波谷位置[17],增大了SAZ化学溶质原子偏析程度,加重了SAZ的腐蚀程度。HAZ沿焊缝厚度向下,晶粒尺寸先增大后减小,所以腐蚀深度也呈现为中间最大,下部次之,上部最小;而HAZ只受热循环的影响,组织发生粗化,沿着板厚方向向下组织粗化程度逐渐减小,因此,HAZ晶间腐蚀深度沿板厚方向向下也逐渐变小。

另一方面,点蚀往往是晶间腐蚀的开端[18],而第二相粒子是铝合金腐蚀的起源。接头各区域中的大尺寸含Fe难溶相电位偏正,与周围Al基体形成微区电偶对,周围Al基体作为阳极被腐蚀,最后导致这些第二相粒子完全脱落,留下腐蚀坑。在焊缝中心区第二相粒子尺寸较小,分布较均匀,留下的大量均匀的小腐蚀坑,减弱了腐蚀介质进一步向金属内部传输。在HAZ中的第二相粒子尺寸最大,腐蚀脱落后留下了较大的腐蚀坑,促进了腐蚀向金属内部进行,腐蚀介质通过腐蚀缺陷不断向基材内部腐蚀。而且,HAZ的板条状组织的晶粒长宽比较大,为晶间腐蚀提供了一个连续的发展空间,使腐蚀产物连续分布,产生锲形力[19],易于发生剥落腐蚀。

表2 第二相粒子成分分析
Table 2 EDS analysis results of second-phase particles in Fig.7(atomic fraction / %)
Element Particle1 Particle2 Particle3
Al 76.76 78.14 78.7
Fe 10.31 10.52 16.68
Cu 5.36 6.94 4.62
Zn 1.32 --- ---
Cr 2.90 2.33 ---
Si 1.91 2.07 ---
Mn 1.17 --- ---

表2 第二相粒子成分分析

Table 2 EDS analysis results of second-phase particles in Fig.7(atomic fraction / %)

3 结论

(1) 7075Al合金FSW焊接接头焊缝中心区组织为细小等轴晶,第二相分布均匀,晶间腐蚀程度最轻;HAZ晶粒尺寸最大,第二相分布均匀性相比最差,晶间腐蚀程度最严重。

(2) 各区晶间腐蚀程度沿板厚向下,NZ逐渐变大,TMAZ先增大后减小,HAZ逐渐变小。

(3) 7075厚板铝合金FSW接头各区沿板厚方向晶粒大小和第二相粒子尺寸及分布存在的差异,是造成接头各区沿板厚方向晶间腐蚀程度不同的主要原因。

The authors have declared that no competing interests exist.

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本文选用Al-Li-Cu-Mg系铝锂合金2060,开展搅拌摩擦焊对接接头显微组织与析出相演变规律研究.搅拌摩擦焊对接接头,呈现典型的母材、热影响区、热机影响区和焊核区四区分布特点.母材为双向板条组织,在α板条中有大量三角形AlCu2Mn化合物析出,但在其它相区,当受到热影响时,该相消失;热影响区组织粗大,热机影响区晶粒受到机械力作用,前进侧拉长,后退侧破碎;焊核区为等轴晶组织,出现了高温析出相AlxCuxMn,均布于整个焊核区域.接头显微硬度在母材区最高,热影响区最低,焊核区低于母材,稳定在115 HV.
DOI:10.11951/j.issn.1005-0299.20140521      Magsci     URL    
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(何建军, 李玉斌, 李盛和. 7075铝合金搅拌摩擦焊接头组织与力学性能研究[J]. 热加工工艺, 2011, 40(19): 142)
利用搅拌摩擦焊方法对7075铝合金板进行焊接,探讨了焊接速度和搅拌头旋转速度等焊接工艺参数对焊缝成形及接头力学性能的影响,并对焊接接头的显微组织进行了分析。结果表明:采用搅拌摩擦焊焊接7075铝合金时,焊接接头具有较好抗拉性能。当旋转速度为750r/min、焊接速度为95mm/min时,焊接接头的强度最高,达到母材抗拉强度(487MPa)的97.4%,并且其伸长率也较高(为3.1%);当旋转速度为950r/min、焊接速度为150mm/min时焊接接头的伸长率最好,为4.7%。总体上看,焊接接头的伸长率和母材相比较低。
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(高辉, 董继红, 张坤. 厚板铝合金搅拌摩擦焊接头组织及性能沿厚度方向的变化规律[J]. 焊接学报, 2014, 35(8): 61)
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[5] Gong W B, Tian H J, Liu W, et al.Mechanical properties change along the thickness direction of thick aluminium alloy 6082-T6 Plate[J]. Rare Met. Mater. Eng., 2012, 41(suppl.2): 854
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(宫文彪, 田红娇, 刘威. 6082-T6铝合金厚板搅拌摩擦焊沿厚度方向性能变化[J]. 稀有金属材料与工程, 2012, 41(增刊2): 854)
采用搅拌摩擦焊(FSW)对厚度为30mm的6082-T6铝合金进行对接单道焊接,焊后沿板厚方向每隔4mm切割试样,切割为7层,研究由焊缝表层至底部性能的变化。结果表明,焊缝整体的抗拉强度为193.8MPa,为母材的60.1%,伸长率为4.50%;第1层到第7层抗拉强度依次为195.9,194.5,193.6,191.9,191.1,187.1和182.4MPa,呈依次降低的趋势,而延伸率则呈上升趋势;对应50%和95%存活率的焊缝疲劳强度也由焊缝表面至底部依次降低,前进侧硬度较低,而焊核区显微硬度由70HV下降到55HV。随厚度增加,轴肩产生的摩擦热自上到下不断降低,是造成中下部工件性能下降的主要原因。
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[6] Canaday C T, Moore M A, Tang W, et al.Through thickness property variations in a thick plate AA7050 friction stir welded joint[J]. Mater. Sci. Eng., 2013, A559: 678
In this study, moderately thick (32 mm) AA7050 plates were joined by friction stir welding (FSW). Various methods were used to characterize the welded joints, including nugget grain size measurements at different locations through the thickness, micro-hardness indentation through nugget, thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ) at different cross section heights, and residual stress measurement using the cut compliance method with full thickness and partial thickness specimens. All testing results are consistent with the presence of a strong gradient in peak temperature through the plate thickness during FSW. Published by Elsevier B.V.
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[7] Zhou P Z, Zhong J, He D Q, et al.Microstructure-property and fracture behavior of friction-stir welded 2519 aluminium alloy plate[J]. Trans. China Weld. Inst., 2006, 27(4): 33
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(周鹏展, 钟掘, 贺地求. 2519厚板搅拌摩擦焊缝组织性能及断裂分析[J]. 焊接学报, 2006, 27(4): 33)
[8] Mao Y Q, Ke L M, Liu F C, et al.Effect of tool pin eccentricity on microstructure and mechanical properties in friction stir welded 7075 aluminum alloy thick plate[J]. Mater. Des., 2014, 62: 334
Four different tools with the pin eccentricity of 0.1mm, 0.2mm, 0.3mm and 0.4mm were designed to friction stir weld 10mm thick AA7075-O plate. The effect of pin eccentricity on microstructure, secondary phase particles transformation and mechanical properties of the joints was investigated. The results show that the nugget area (ANZ) increases firstly and then decreases with increasing the pin eccentricity. When the pin with 0.2mm eccentricity is applied, theANZis the largest; meanwhile the grains size is the smallest which is about 3μm and secondary phase particles are the most dispersive in nugget zone compared with other tools. While the grains are coarsened to 7–11μm as the eccentricity is more than 0.4mm, some coarse hardening particles get to cluster in the thermo-mechanically affected zone. The joints produced by the pin with 0.2mm eccentricity perform the highest tensile strength and elongation, which is attributed to better interfaces, finer grains and more dispersive secondary phase particles.
DOI:10.1016/j.matdes.2014.05.038      URL     [本文引用:]
[9] Zhou P Z, Li D H, He D Q, et al.Through-thickness diversity of properties in friction stir welded 2219-T87 thick aluminum alloy plate[J]. Trans. China Weld. Inst., 2007, 28(10): 5
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(周鹏展, 李东辉, 贺地求. 2219-T87厚板搅拌摩擦焊沿厚度方向的性能差异[J]. 焊接学报, 2007, 28(10): 5)
采用搅拌摩擦焊方法对35mm厚的2219-T87高强铝合金进行了单道对接焊。结果表明,焊缝的抗拉强度可达274MPa,且断裂于焊核区。用螺旋形搅棒焊接厚板时,焊接速度较慢,焊核区上部组织因温度较高,同时在螺旋产生的向下泵吸作用下,会产生比较粗大的疏松组织;而焊核区中下部组织则因温度较低,并在螺旋推力作用下形成比较细小的致密组织。焊核区上部的显微硬度存在一个下陷区,而中下部的显微硬度变化较为平缓。焊接厚度过大时,螺旋搅棒的泵吸作用引起焊核区上部组织疏松,并使其显微硬度下陷,是造成厚板搅拌摩擦焊缝强度不高且断裂于焊核区的原因。
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The pitting corrosion of different positions (Top, Middle and Bottom) of weld nugget zone (WNZ) along thickness plate in friction stir welded 2219-O aluminum alloy in alkaline chloride solution was investigated by using open circuit potential, cyclic polarization, scanning electron microscopy and atomic force microscope. The results indicate that the material presents significant passivation, the top has highest corrosion potential, pitting potential and re-passivation potential compared with the bottom and base material. With the increase of traverse speed from 60 to 100 mm/min or rotary speed from 500 to 600 rpm, the corrosion resistance decreases.
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[13] Lin G Y.Fundamental research related to the fabrication technology for high quality thick plates of 7×75 series aluminum alloys [D]. Changsha: Central South University, 2006(林高用. 高性能7×75系铝合金厚板加工技术相关基础研究 [D]. 长沙: 中南大学, 2006)
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[15] Zhou K, Wang B, Zhao Y, et al.Corrosion and electrochemical behaviors of 7A09 Al-Zn-Mg-Cu alloy in chloride aqueous solution[J]. Trans. Nonferrous Met. Soc. China, 2015, 25: 2509
The corrosion and electrochemical behaviors of 7A09 Al–Zn–Mg–Cu alloy were investigated in 3.5% NaCl (mass fraction) solution using complementary techniques such as scanning electron microscopy (SEM), metallographic microscopy and electrochemical measurements. The results show that both pitting corrosion from or around the intermetallic particles and intergranular corrosion are observed after the immersion test due to the inhomogeneous nature of the microstructure of the 7A09 alloy. The preferential dissolution of the anodic Cu-depleted zone along grain boundaries is believed to be the possible cause of intergranular corrosion. The passivation and depassivation of this alloy show significant dependence of immersion time, owing to the formation and dissolution of various passive films on the sample surfaces. Furthermore, the corrosion process and corrosion mechanism were also analyzed.
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The effect of annealing conditions producing various grain sizes on the intergranular corrosion behavior of high-strength aluminum alloy type 7075-T6 was investigated using electrochemical polarization techniques. Aluminum alloy specimens with large grain size exhibited lower breakdown potentials in deaerated 0.5 M NaCl solution. The breakdown potentials decreased with increasing grain size. Microscopic observations of the exposed surfaces during potentiostatic polarization testing showed that the coarse grain structure promotes intergranular crack growth.
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[17] Yang B C, Yan J H, Sutton M A, et al.Banded microstructure in AA2024-T351 and AA2524-T351 aluminum friction stir welds: Part I. Metallurgical studies[J]. Mater. Sci. Eng., 2004, A364: 55
Results from careful investigations of the banded microstructure observed on horizontal transverse cross-sections in all AA2024-T351 and AA2524-T351 aluminum friction stir weld (FSW) joints indicate the presence of periodic variations in (a) the size of equiaxed grains, (b) micro-hardness, and (c) concentration of base metal impurity particles (i.e. constituent particles) that correlate with the observed band spacing. The latter trend is more distinct in AA2024-T351, which has a higher volume fraction of constituent particles resulting in easily recognizable particle rich regions on horizontal cross-sections near the mid-thickness of the joint and well-defined variations in hardness. In AA2524, the trends are more muted but clearly visible. Results from recent numerical simulations of the FSW process enable interpretation of the trends in grain size along the weld centerline in terms of the time鈥搕emperature cycle experienced by the material. Specifically, the AA2524 FSW joints having low power and high input energy (i.e. the slow FSW), exhibit micron-size grain structure across both bands. Conversely, the fast and medium FSW in AA2524 have higher maximum temperatures, and a corresponding six-fold increase in grain size.
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(李朝兴, 徐静, 李劲风. 不同时效制度7075铝合金力学性能及腐蚀性能综合比较研究[J]. 铝加工, 2009, (5): 10)
采用室温拉伸和剥落腐蚀、晶间腐蚀方法,结合金相观察和透射电子显微镜(TEM)观察,研究了T6、RRA、二次时效(T616)、双极过时效(T73)及高温预析出(HTPP)对7075铝合金力学性能及腐蚀性能的影响及其影响机制.
[19] Luo Z J, Yuan G C, Huang Z T, et al.Exfoliation corrosion resistance of 5083 aluminum alloy welds processed by friction stir welding[J]. Mater. Res. Appl., 2015, 9: 107
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(骆志捷, 袁鸽成, 黄泽涛. 5083铝合金搅拌摩擦焊缝的剥落腐蚀性能[J]. 材料研究与应用, 2015, 9: 107)
采用溶液浸泡法研究了5083铝合金及其搅拌摩擦焊焊缝的剥落腐蚀性能。借助光学显微镜、扫描电子显微镜、透射电子显微镜及电化学工作站分析了焊缝和母材的微观组织、剥落腐蚀形貌、极化曲线及电化学阻抗谱。结果表明,浸泡腐蚀后焊缝出现轻微点蚀,而母材点蚀严重并显现局部起皮剥落,腐蚀裂纹沿晶开裂;腐蚀评级和电化学阻抗谱均可说明焊缝的耐剥落腐蚀性能好于母材,同时极化曲线也说明焊缝的腐蚀敏感性低于母材。带状晶粒等轴化,棒状的第二相细化及分布均匀化,位错密度的减小是焊缝耐剥落腐蚀性能强于母材的主要原因。
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关键词(key words)
搅拌摩擦焊
厚板
7075铝合金
晶间腐蚀
第二相

FSW
heavy plate
7075 Al-alloy
intergranular corrosion
the second phase

作者
刘德强
柯黎明
徐卫平
邢丽
毛育青

LIU Deqiang
KE Liming
XU Weiping
XING Li
MAO Yuqing